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Short_Version_Aura_OMI_Level2_OMAERUV.003.cdl
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Short_Version_Aura_OMI_Level2_OMAERUV.003.cdl
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// Purpose: CDL equivalent of the Level 2 OMI OMAERUV product files in
// HDF5-based HDF-EOS format archived in NASA GES DISC. The files shows how a
// Level 2 swath data file is usually encoded in the HDF-EOS convention and how
// they can be converted to a netCDF structure with CF convention.
// A short version of the "Aura_OMI_Level2_OMAERUV.003.cdl" by removing
// variables with similar dimensionalities, but the viewing/solar geometry
// variables and one quality screning variable are still included to show the
// variety of information a L1/L2 swath files commonly include. File and swath
// level attrubutes are removed as they mostly likely will be replaced with CF-
// compliant global attributes.
// Some additional changes are made to make the file more CF-compliant:
// 1) added a new attribute, "coordinates", for data variables to identify
// associated coordinate variables.
// 2) removed fill/missing value and scale/offset attributes from coordinate
// variables.
// 3) changed attributes names, such as "ScaleFactor" to "scale_factor", and
// "title" to "long_name".
// 4) changed unit values, such as "deg" to "degree_east" or "degree_north".
// 5) removed the unit attribute when its value is "NoUnits".
// 6) removed most file/swath level attributes, and changed/added three
// CF global attributes.
// 7) removed from the integer variables the scale/offset when their values are
// equal to 1.0 and 0.0, respectively. They will otherwise cause compliancy
// error. Actually they are also not needed for variables of the other data
// types.
// This file can be converted to a nc4 file, but not a nc3 data file using
// ncgen because the ubyte and ushort variables are not supported by netCDF3:
// ncgen -k netCDF-4 -b Short_Version_Aura_OMI_Level2_OMAERUV.003.cdl
// Known CF-compliancy issues when this file is converted to a "CF-convention
// netCDF":
// 1) data values are not present in the file.
// 2) Missing the "units" attribute for some data variables whose values are of
// unitless. I am not sure what's the unit value for unitless data values
// such as aerosol optical depth. In the HDF-EOS communit, NoUnits is
// commonly used but it's not CF-compliant value.
// Contact: Wenli Yang, GES DISC, wenli.yang@nasa.gov
netcdf {
// Template source:
// ftp://aurapar2u.ecs.nasa.gov/data/s4pa/Aura_OMI_Level2/OMAERUV.003/2015/001/OMI-Aura_L2-OMAERUV_2015m0101t0130-o55655_v003-2015m0107t202422.he5
:Title = "OMI Aerosol Optical Depth and Single Scattering Albedo" ;
:Conventions = "CF-1.6.0" ;
:Institution = "NASA-GSFC" ;
// Dimensions
// Note: in HDF-EOS, the dimension names are stored in the structure metadata.
// If one uses ncdump to dump a HDF-EOS file, the dimension names in its
// structure will not be recognized by ncdump and the names will be shown as
// phony_dim_0, phony_dim_1, etc.
dimensions:
nTimes = 1644 ; // number of time steps in a swath, the same as number of along track data points, "Track".
nXtrack = 60 ;
nLayers = 5 ; // number of height layers. In CF, it should be named "height"
nWavel = 3 ; // number of wavelengths. A more common name for this is "Band"
variables:
// Coordinate variables.
// They are originally included in the "/HDFEOS/SWATHS/Aerosol NearUV
// Swath/Geolocation Fields/" group. Notice that 1) two time values are
// provided in this product, one UTC time and the other seconds in the day; 2)
// the wavelength coordinate variable is not placed in the "geolocation
// fields" but in the "data fields" because they are not "geo"; 3) the values
// for the "nLayer" coordinate is not included in this product. It should be
// provided to compliant with CF.
float latitude(nTimes, nXtrack) ;
latitude:units = "degrees_north" ;
latitude:long_name = "Geodetic latitude (deg)" ;
latitude:UniqueFieldDefinition = "Aura-Shared" ;
float longitude(nTimes, nXtrack) ;
longitude:units = "degrees_east" ;
longitude:long_name = "Geodetic longitude (deg)" ;
longitude:UniqueFieldDefinition = "Aura-Shared" ;
double time(nTimes) ;
time:units = "s" ;
time:long_name = "Time at Start of Scan (s, TAI93)" ;
time:UniqueFieldDefinition = "Aura-Shared" ;
// the wavelength variable is placed in the "Data fields"
float wavelength(nWavel) ;
wavelength:units = "nm" ;
wavelength:long_name = "Wavelength" ;
wavelength:UniqueFieldDefinition = "OMI-Specific" ;
// An additional coordinate variable for height, float height(nLayers),
// should be included to compliant with CF. The height values are presented
// in the swath attribute as shown above and can be readily used for a height
// coordinate variable, as the following:
float height(nLayers);
height:units = "km" ;
height:long_name = "vertical distance above the surface" ;
height:standard_name = "height" ;
height:positive = "up";
height:axis = "Z";
// Data variables.
// They are originally included in the "/HDFEOS/SWATHS/Aerosol NearUV
// Swath/Data Fields/" group.
float AIRSL3COvalue(nTimes, nXtrack) ;
AIRSL3COvalue:coordinates = "time latitude longitude" ;
AIRSL3COvalue:_FillValue = -9999.f ;
AIRSL3COvalue:missing_value = -9999.f ;
AIRSL3COvalue:add_offset = 0.f ;
AIRSL3COvalue:scale_factor = 1.f ;
AIRSL3COvalue:long_name = "AIRS CO L3 data" ;
AIRSL3COvalue:UniqueFieldDefinition = "OMI-Specific" ;
float AerosolAbsOpticalDepthVsHeight(nTimes, nXtrack, nLayers, nWavel) ;
AerosolAbsOpticalDepthVsHeight:coordinates = "time latitude longitude height wavelength" ;
AerosolAbsOpticalDepthVsHeight:_FillValue = -1.267651e+30f ;
AerosolAbsOpticalDepthVsHeight:missing_value = -1.267651e+30f ;
AerosolAbsOpticalDepthVsHeight:add_offset = 0.f ;
AerosolAbsOpticalDepthVsHeight:scale_factor = 1.f ;
AerosolAbsOpticalDepthVsHeight:long_name = "Aerosol Absorption Optical Depth (tau_abs) at 5 levels" ;
AerosolAbsOpticalDepthVsHeight:UniqueFieldDefinition = "OMI-Specific" ;
float FinalAerosolAbsOpticalDepth(nTimes, nXtrack, nWavel) ;
FinalAerosolAbsOpticalDepth:coordinates = "time latitude longitude wavelength" ;
FinalAerosolAbsOpticalDepth:_FillValue = -1.267651e+30f ;
FinalAerosolAbsOpticalDepth:missing_value = -1.267651e+30f ;
FinalAerosolAbsOpticalDepth:add_offset = 0.f ;
FinalAerosolAbsOpticalDepth:scale_factor = 1.f ;
FinalAerosolAbsOpticalDepth:long_name = "Best Aerosol Absorption Optical Depth (tau_abs)" ;
FinalAerosolAbsOpticalDepth:UniqueFieldDefinition = "OMI-Specific" ;
ushort MeasurementQualityFlags(nTimes) ;
MeasurementQualityFlags:coordinates = "time" ;
MeasurementQualityFlags:_FillValue = 65535US ;
MeasurementQualityFlags:missing_value = 65535US ;
MeasurementQualityFlags:long_name = "Measurement Quality Flags" ;
MeasurementQualityFlags:UniqueFieldDefinition = "OMI-Specific" ;
// Additional variables which are originally included in the swath's
// geolocation fields but should be considered data variables in CF. Notice
// that in HDF-EOS, solar and viewing geometry variables are traditionally
// placed in geolocation fields (e.g., in MODIS L1B).
float RelativeAzimuthAngle(nTimes, nXtrack) ;
RelativeAzimuthAngle:coordinates = "time latitude longitude" ;
RelativeAzimuthAngle:_FillValue = -1.267651e+30f ;
RelativeAzimuthAngle:missing_value = -1.267651e+30f ;
RelativeAzimuthAngle:units = "degrees" ;
RelativeAzimuthAngle:long_name = "Relative (sun + 180 - view) Azimuth Angle (deg)" ;
RelativeAzimuthAngle:UniqueFieldDefinition = "OMI-Specific" ;
float SolarZenithAngle(nTimes, nXtrack) ;
SolarZenithAngle:coordinates = "time latitude longitude" ;
SolarZenithAngle:_FillValue = -1.267651e+30f ;
SolarZenithAngle:missing_value = -1.267651e+30f ;
SolarZenithAngle:units = "degrees" ;
SolarZenithAngle:long_name = "Solar Zenith Angle (deg)" ;
SolarZenithAngle:UniqueFieldDefinition = "Aura-Shared" ;
float ViewingZenithAngle(nTimes, nXtrack) ;
ViewingZenithAngle:coordinates = "time latitude longitude" ;
ViewingZenithAngle:_FillValue = -1.267651e+30f ;
ViewingZenithAngle:missing_value = -1.267651e+30f ;
ViewingZenithAngle:units = "degrees" ;
ViewingZenithAngle:long_name = "Viewing Zenith Angle (deg)" ;
ViewingZenithAngle:UniqueFieldDefinition = "OMI-Specific" ;
ushort GroundPixelQualityFlags(nTimes, nXtrack) ;
GroundPixelQualityFlags:coordinates = "time latitude longitude" ;
GroundPixelQualityFlags:_FillValue = 65535US ;
GroundPixelQualityFlags:missing_value = 65535US ;
GroundPixelQualityFlags:long_name = "Ground Pixel Quality Flags" ;
GroundPixelQualityFlags:UniqueFieldDefinition = "OMI-Specific" ;
// the terrain pressure is actually a data variable. It contains the
// the pressure value at the terrain surface. The variable is not
// used as coordinate for any other variables.
float TerrainPressure(nTimes, nXtrack) ;
TerrainPressure:coordinates = "time latitude longitude" ;
TerrainPressure:_FillValue = -1.267651e+30f ;
TerrainPressure:missing_value = -1.267651e+30f ;
TerrainPressure:add_offset = 0.f ;
TerrainPressure:scale_factor = 1.f ;
TerrainPressure:units = "mbar" ;
TerrainPressure:long_name = "Terrain Pressure (mbar)" ;
TerrainPressure:UniqueFieldDefinition = "Aura-Shared" ;
// Data values are not included in this example
data:
}